1. Field of the Invention
The present invention relates to systems and methods for remote chemical sensing. More specifically, the present invention relates to systems and methods for laser pulse slicing and separately optimized wavelength shifting by nonlinear crystal techniques.
2. Description of the Related Art
There are numerous applications for which a system or method for remote detection of airborne chemicals is highly desirable. For military applications, by way of example, there is need for a remote chemical sensing system to detect for a deployment of chemical weapons. For commercial and industrial applications, there is a need for a remote chemical sensing system to detect pollutants, monitor processes and etc.
While many techniques have been considered for remote chemical detection, lasers have been found to be most effective. The remote detection of airborne chemicals using lasers is currently well established. The transmitter typically used for this application is a pulsed carbon dioxide (CO2) laser because of its high energy capability in the 9-11 xcexcm band where most chemicals have strong absorption features.
Unfortunately, certain chemicals and chemical weapons such as mustard gas (and others yet to be deployed) cannot be readily detected due to their very weak absorption in the band 9-11 xcexcm band. However, mustard gas does have a strong absorption feature at 8.3 xcexcm by which it could be easily detected. Therefore, shifting of the CO2 laser wavelength to 8.3 xcexcm band by combined second harmonic generation (SHG) and optical parametric oscillation (OPO) in nonlinear crystals is being pursued.
The typical CO2 laser waveform used for this purpose is a pulse characterized by a short spike followed by a long, low intensity tail, typical of a transversely excited atmospheric (TEA) laser. The tail can contain 60% of the total pulse energy. The present approach to SHG/OPO shifting is to tailor the crystals and pump beam parameters to convert the spike without regard for the tail which, because of its much lower intensity, is not converted, wasting that portion of the total pulse energy.
Solid state lasers have been used for chemical detection; however, solid state lasers also do not generally lase in the 8.3 xcexcm band. Efforts to shift the output wavelength of solid state lasers have met with some success. The output energies have been low and therefore the ranges have been restricted. In addition, beam quality has been poor. That is, the bandwidth of the output lines have been too broad to the extent that achieving sufficient sensing specificity has been difficult. This has limited the accuracy by which chemicals can be detected with solid state lasers.
Hence, there is a need in the art for a more effective system or method for remotely sensing new chemicals and other previously undetectable chemicals.
The need in the art is addressed by the wavelength conversion system and method of the present invention. The inventive system is adapted to receive a beam of input energy having a first portion with a first wavelength and a first spatial and/or temporal intensity profile and a second portion with the first wavelength and a second spatial and/or temporal intensity profile. The inventive system includes a mechanism for directing the first portion of the input beam along a first processing path and a second portion thereof along a second processing path. A first arrangement is disposed in the first path for shifting the wavelength of the first portion of the input energy from the first wavelength to a second wavelength. The first arrangement is optimized for the first spatial and/or temporal intensity profile of the first portion of the energy. A second arrangement is disposed in the second path for shifting the wavelength of the second portion of the input energy from the first wavelength to the second wavelength. The second arrangement is optimized for the second spatial and/or temporal intensity profile of the second portion of the energy.
In the illustrative application, the input energy is a pulse of electromagnetic energy such as might be supplied by a carbon-dioxide laser. In the illustrative embodiment, the mechanism for directing the first portion of the input beam along a first processing path and a second portion thereof along a second processing path includes an electro-optic switch adapted to rotate a polarization state of at least one of the portions of the input energy such that the first portion has a first polarization state and the second portion has a second polarization state. The output of the switch is input to a first polarizer which directs the first portion along the first path and the second portion along the second path.
In the illustrative embodiment, the first and second arrangements include a second harmonic generator that shifts the pulse first portion of the energy from the first wavelength to an intermediate wavelength. The first and second arrangements further include an optical parametric oscillator for shifting the pulse second portion of the energy from the intermediate wavelength to the second wavelength. A second polarizer is included for combining the first and second portions into a single beam.